Electronic circuits, such as integrated microcircuits, are used in a variety of products, from automobiles to microwaves to personal computers. Designing and fabricating microcircuit devices typically involves many steps, sometimes referred to as the “design flow.” The particular steps of a design flow often are dependent upon the type of microcircuit, its complexity, the design team, and the microcircuit fabricator or foundry that will manufacture the microcircuit. Typically, software and hardware “tools” verify the design at various stages of the design flow by running software simulators and/or hardware emulators, these steps aid in the discovery of errors in the design and allow the designers and engineers to correct or otherwise improve the design.
Several steps are common to most design flows. Initially, the specification for a new circuit is transformed into a logical design, sometimes referred to as a register transfer level (RTL) description of the circuit. With this logical design, the circuit is described in terms of both the exchange of signals between hardware registers and the logical operations that are performed on those signals. The logical design typically employs a Hardware Design Language (HDL), such as the Very high speed integrated circuit Hardware Design Language (VHDL). The logic of the circuit is then analyzed, to confirm that it will accurately perform the functions desired for the circuit. This analysis is sometimes referred to as “functional verification.”
After the accuracy of the logical design is confirmed, it is converted into a device design by synthesis software. The device design, which is typically in the form of a schematic or netlist, describes the specific electronic devices (such as transistors, resistors, and capacitors) that will be used in the circuit, along with their interconnections. This device design generally corresponds to the level of representation displayed in conventional circuit diagrams. The relationships between the electronic devices are analyzed, to confirm that the circuit described by the device design will correctly perform the desired functions. This analysis is sometimes referred to as “formal verification.” Additionally, preliminary timing estimates for portions of the circuit are often made at this stage, using an assumed characteristic speed for each device, and incorporated into the verification process.
Once the components and their interconnections are established, the design is again transformed, this time into a physical design that describes specific geometric elements. This type of design often is referred to as a “layout” design. The geometric elements, which typically are polygons, define the shapes that will be created in various materials to manufacture the circuit. Typically, a designer will select groups of geometric elements representing circuit device components (e.g., contacts, gates, etc.) and place them in a design area. These groups of geometric elements may be custom designed, selected from a library of previously-created designs, or some combination of both. Lines are then routed between the geometric elements, which will form the wiring used to interconnect the electronic devices. Layout tools (often referred to as “place and route” tools), such as Mentor Graphics' IC Station or Cadence's Virtuoso, are commonly used for both of these tasks.
IC layout descriptions can be provided in many different formats. The Graphic Data System II (GDSII) format is a popular format for transferring and archiving 2D graphical IC layout data. Among other features, it contains a hierarchy of structures, each structure containing layout elements (e.g., polygons, paths or poly-lines, circles and textboxes). Other formats include an open source format named Open Access, Milkyway by Synopsys, Inc., EDDM by Mentor Graphics, Inc., and the more recent Open Artwork System Interchange Standard (OASIS) proposed by Semiconductor Equipment and Materials International (SEMI). These various industry formats are used to define the geometrical information in IC layout designs that are employed to manufacture integrated circuits. Once the microcircuit device design is finalized, the layout portion of the design can be used by fabrication tools to manufacturer the device using a photolithographic process.
There are many different fabrication processes for manufacturing a circuit, but most processes include a series of steps that deposit layers of different materials on a substrate, expose specific portions of each layer to radiation, and then etch the exposed (or non-exposed) portions of the layer away. For example, a simple semiconductor circuit could be manufactured by the following steps. First, a positive type epitaxial layer is grown on a silicon substrate through chemical vapor deposition. Next, a nitride layer is deposited over the epitaxial layer. Then specific areas of the nitride layer are exposed to radiation, and the exposed (or non-exposed areas are etched away, leaving behind exposed areas on the epitaxial layer, (i.e., areas no longer covered by the nitride layer). Specific shapes or patterns on these exposed areas then are subjected to a diffusion or ion implantation process. This causes dopants, (for example, phosphorus) to enter the exposed epitaxial layer and form negative wells. This process of depositing layers of material on the substrate or subsequent material layers, and then exposing specific patterns to radiation, etching, and dopants or other diffusion processes, is repeated a number of times. This series of steps allows the different physical layers of the circuit to be manufactured.
Each time that a layer of material is exposed to radiation, a mask must be created to expose only desired areas to the radiation, and to protect the other areas from exposure. Each mask is created from circuit layout data. That is, the geometric elements described in layout design data define the relative locations or areas of the circuit device that will be exposed to radiation through the mask. A mask or reticle writing tool is used to create the mask or reticle based upon the layout design data, after which the mask can be used in a photolithographic process.
As designers and manufacturers continue to increase the number of circuit components in a given area and/or shrink the size of circuit components, the shapes reproduced on the substrate (and thus the shapes in the mask) become smaller and are placed closer together. This reduction in feature size increases the difficulty of faithfully reproducing the image intended by the layout design onto the substrate. This difficulty often results in defects where the intended image is not accurately “printed” onto the substrate during the photolithographic process, creating flaws in the manufactured device.
To address this problem, one or more resolution enhancement techniques are often employed. Examples of various resolution enhancement techniques are discussed in “Resolution Enhancement Technology: The Past, the Present, and Extensions for the Future,” Frank M. Schellenberg, Optical Microlithography XVII, edited by Bruce W. Smith, Proceedings of SPIE Vol. 5377, which article is incorporated entirely herein by reference. One of these techniques, radiation amplitude control, is often facilitated by modifying the layout design data employed to create the lithographic mask. One way to implement this technique, for example, is to adjust the edges of the geometric elements in the layout design so that the mask created from the modified layout data will control the radiation amplitude in a desired way during a lithographic process. The process of modifying the layout design data in this manner is often referred to as “optical proximity correction” or “optical process correction” (OPC).
As previously noted, a layout design is made up of a variety of geometric elements. In a conventional optical proximity correction process, the edges of the geometric elements (which are typically polygons) are fragmented, and the edge fragments are rearranged or edited to effect the desired modifications. The edge fragments are typically reconfigured to ensure the design conforms to manufacturing constraints, such as the minimum size of a fragment or the minimum proximity between adjacent fragments. Additionally, features that will increase the fidelity of the photolithographic process are often added to the design. For example, some optical proximity correction processes will reconfigure the edge fragments of a polygon to create serifs at one or more corners.
As microcircuit devices get smaller and more complex, additional manufacturing processes are often employed to fabricate the devices. For example, chemical-mechanical planarization is often used both to flatten the topography and to bring certain features of the topography into the depth of field of the lithographic process. Additionally, as described above, many etching and diffusing steps are performed during the fabrication process. These additional processes provide opportunities for other manufacturing defects to manifest themselves. Accordingly, any manufacturing defect, whether due to the lithographic process or some other process such as chemical-mechanical planarization will be referred to herein as a “fault,” a “potential fault,” or a “potential manufacturing fault.”
Due to the various processes involved in manufacturing, it is increasingly difficult to predict and simulate all potential faults that could manifest during fabrication. Prior to a layout design being finalized and a mask created, the layout must be examined to ensure that the design does not have potential manufacturing faults. If there are potential faults, then these potential faults must be corrected. While performing optical proximity correction on layout design data can improve the fidelity of the lithographic process, the correction process can be expensive in terms of both computing resources and processing time. Layout designs can be very large, and even one layout data file for a single layer of a field programmable gate array may be approximately 58 gigabytes. Accordingly, performing even a single optical proximity correction process on a design is computationally intensive. Repeating an optical proximity correction process to correct remaining potential faults then only adds to the time required to finalize the layout design. On the other hand, manually correcting potential faults is very time consuming as well. The time required for performing optical proximity correction only increases as the feature size of designs decrease and as the number of features in a given design increases. Additionally, potential faults due to other procedures in the manufacturing process, for example chemical-mechanical planarization, require correction procedures in addition to optical proximity correction. This further complicates and adds costs in the form of computing resources and engineer time to address.